Acute disseminated encephalomyelitis (ADEM) is a neurological disorder characterized by brief but widespread attacks of inflammation (swelling) in the brain and spinal cord that damages myelin. Myelin is the whitish protective coating over nerves that helps with electrical nerve signaling. ADEM often appears following a viral or bacterial infection and sometimes (rarely) following a vaccination. It is sometimes misdiagnosed as the first severe attack of multiple sclerosis, since the symptoms and appearance of damage to the white matter may be similar. ADEM may be an autoimmune condition, in which the body's immune system mistakenly identifies and attacks healthy cells and tissue. Symptoms include:

• Loss of vision in one or both eyes due to inflammation of the optic nerve
• Weakness that may be severe
• Difficulty coordinating intended (voluntary) movement such as walking
• Rapid onset of fever
• Some degree of impairment of consciousness, perhaps as severe as coma

ADEM can occur in people of any age, but it most often occurs in young children.

Treatment is targeted at suppressing the immune system and inflammation in the brain using anti-inflammatory drugs such as corticosteroids. Some individuals may need plasma exchange (plasmapheresis) or immunoglobulin therapy. Most people with ADEM being to recover within days after treatment, and many will recover completely within six months. In rare instances, ADEM can be fatal.